This invention relates generally to flowmeters of the vortex-shedding type whose obstacle assembly includes a deflectable section excited into vibration by fluidic oscillations, and more particularly to a relatively small vortex meter of the external sensor type which is substantially insensitive to acceleration forces whereby the meter provides accurate readings regardless of shock waves or other forces other than fluidic oscillations which seek to excite the deflectable section.
In many industrial processes, one must be able to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic fluidic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed, these being known as Karman vortex streets. The periodicity at which vortices are shed in a Karman vortex street is a function of flow rate.
In Burgess U.S. Pat. No. 3,888,120, the obstacle assembly mounted in a flow tube through which the fluid to be metered is conducted is formed by a front section fixedly mounted across the tube and a rear section cantilevered from the front section by means of a flexible beam to define a gap serving to trap the Karman vortices. Because the rear section is deflectable, it is excited into mechanical vibration by the vortices at a rate whose frequency is proportional to fluid flow.
In a 10 LV 100 model vortex meter manufactured by the Fischer & Porter Co. of Warminster, Pa. (the assignee herein) and in commercially-available meters operating on similar principles, the relatively heavy deflectable section of the obstacle assembly which is suspended from a single beam has freedom of motion in two planes. The deflectable section is free to move from side-to-side with respect to its neutral position in a lateral plane, hereinafter referred to as the X plane. It can also move up-and-down with respect to its neutral position in an axial plane, hereinafter referred to as the Y plane.
When the deflectable section moves in either the X or Y plane, bending of the beam occurs. This bending action imposes a strain on a beam-mounted sensor of the strain gauge type to generate an output signal which reflects the extent of movement and the frequency or repetition rate thereof.
In normal operation, the flow of fluid past the obstacle assembly produces vortex shedding, giving rise to fluidic forces which alternate from one side of the deflectable section to the other, thereby causing this section to vibrate in the X plane at a frequency proportional to flow rate.
When, however, the vortex meter is installed in a pipeline, it may in certain cases be subject to acceleration forces. Thus in a field installation in which the meter is included in the piping of a large industrial process system which incorporates heavy machinery or explosive activity, vibratory or shock wave forces may be transmitted by the piping to the meter. These extraneous forces are picked up by the meter and cause the beam-supported deflectable section to behave in a manner comparable to the spring-mounted inertial mass of an accelerometer to produce an output signal that is a function of acceleration forces applied in the X and Y planes.
Hence in a field installation in which the vortex meter is exposed to acceleration forces, the output signal will not accurately reflect flow rate, for this signal is a composite which includes a spurious acceleration component.
In the copending application of Herzl (common assignee) Ser. No. 668,458, filed Mar. 19, 1976, and entitled "Accleration-Proof Vortex Type Flowmeter," now U.S. Pat. No. 4,003,251 there is disclosed a meter whose structure is rendered immune to acceleration forces, the deflectable section of the obstacle assembly being cantilevered from a fixed section thereof by two spaced upper and lower beams whose physical characteristics are such as to cause this section to swing about a fulcrum.
A significant feature of the meter disclosed in the copending Herzl application is that the deflectable section of the obstacle assembly, instead of vibrating from side to side with respect to its neutral position as in a conventional meter, is effectively fulcrumed to swing back and forth on one of the beams acting as a torsion bar. The physical characteristics of the upper beam are such that this beam is weak and therefore bendable in the X plane defined by side-to-side movement of the deflectable section with respect to its neutral position and is stiff and unbendable in the Y plane defined by up and down movement, the upper beam being weak torsionally. The physical characteristics of the lower beam are such that this beam is stiff in both the Y and X planes and is weak torsionally. Thus the lower beam cannot be bent by the deflectable section but can only be twisted thereby.
The two-beam obstacle assembly structure disclosed in the copending Herzl application is best suited for inclusion in relatively large flowmeters of the vortex-shedding type, but it is not feasible to incorporate this two-beam arrangement in a small vortex flowmeter which includes an external-sensor sensing system.
An example of an external-sensor vortex meter is disclosed in the 1976 patent to Herzl, U.S. Pat. No. 3,946,608, wherein the vibrations of a deflectable section of an obstacle assembly disposed within a flow tube are mechanically transmitted to a point outside the meter by a rod extending through the beam which cantilevers the deflectable section from a front section fixedly mounted across the tube, the rod being linked to a probe at right angles to the rod which extends through the fixed section and terminates in an external coupling head. This head is engageable by a force sensor which functions to convert the coupling head vibrations into a corresponding electrical signal whose frequency is proportional to flow rate.
An external-sensor vortex meter of the type disclosed in the Herzl patent is also subject to acceleration forces which produce spurious readings, but the nature of the external-sensor sensing system incorporated in such meters and the relatively small size of such meters are such as to preclude, as a practical matter, the expedients included in the above-identified copending Herzl application to render the meter immune to acceleration forces.